1. Field of the Invention
This invention relates generally to methods and apparatus for estimating the flow rate of blood from the heart, and more particularly to an improved "thermal dilution method" of making estimates and improved apparatus for use in that method.
2. Prior Art
A currently usual technique, called the "thermal dilution method," for estimating the rate of flow of blood from a living patient's heart is to inject a bolus of cold fluid, which in this document will be called "injectate," into the entrance of the heart, and to record the temperature of the thoroughly mixed blood downstream from the heart. The temperature recording proceeds as a function of time, and over an interval necessary to estimate the mean temperature change--normally less than twenty seconds.
The average temperature of the injectate is estimated by measurement at the point of entry into the catheter, or by actually measuring the temperature of the injectate "bath," in which the injectate syringe is cooled, or by forcing the injectate to asssume the temperature of melting ice by long "soaking" in a mixture of water and ice.
The temperature sensors most commonly used for the required blood-temperature measurements have been thermistors, since they have a high sensitivity--on the order of 10,000 microvolts per degree Celsius. Precise precalibrated thermistors, however, are expensive, since they are employed in single-use (or disposable) equipment.
Thermocouples have not been as widely used, for two reasons. First, their output is much lower--on the order of fifty microvolts per degree. Second, they have been used with costly and inconvenient thermostatted reference-temperature devices, which will be discussed further below.
Thermocouples have the advantage of low cost, since they consist of a pair of wires that can be cheaply purchased in very large quantities from a single "melt" batch of identical Seebeck-coefficient alloy. Furthermore, since the output voltage calibration of thermocouples depends solely upon the materials of their wires, only sporadic calibration is necessary in the manufacturing process.
In view of these unrealized advantages, it is worthwhile to explore the previously mentioned disadvantages of thermocouples in greater detail, to understand their causes, ramifications, and possible mitigations.
First, the low voltage output of thermocouples requires high amplification to provide usable signals. High amplification in turn has been associated with either poor stability, and/or frequent adjustment.
Second, the reference-temperature device--which in this document will be called the "reference heat sink" is necessary because of the well-known requirement for using thermocouples in pairs. A thermocouple consists of two wires of dissimilar material joined at a "junction" and respectively extending away from the junction. Two identical thermocouples connected together at one point by their similar-metal wires, form a "thermocouple pair." One junction of the pair is designated the "sensing" junction and the other the "reference" junction.
The voltage output of such a thermocouple pair is the open-circuit voltage measurable between the nonjoined similar-metal wires of the thermocouple pair. This voltage is approximately proportional to the temperature difference between the sensing and reference junctions, especially in a narrow temperature range of interest in this field. Consequently the sensing junction is placed in a location where the temperature is to be sensed, and the reference junction conventionally is placed in a reference heat sink of precisely known temperature. The temperature to be measured is then found as the difference between (1) the voltage output of the pair, multiplied by the calibration factor in degrees per volt, and (2) the known reference temperature of the reference heat sink.
It is common to use for the reference heat sink an insulated container filled with pure ice and pure water, so that the reference junction is maintained at exactly zero degrees Celsius.
It is inconvenient to use pure ice and water. Moreover, there is a large temperature difference between the junctions--for example, 37 Celsius degrees between blood in the human body and the freezing point of water. Therefore the change in thermocouple voltage output for small temperature deviations is very small compared to the total voltage across the thermocouples. As is well known, such a relationship adds to measurement inaccuracy.
For example, where the blood is at 37 degrees Celsius, the output from a thermocouple pair referred to icewater is approximately 1,480 microvolts. A typical peak change in blood temperature when injectate is introduced is -0.5 degree Celsius, leading to a change in thermocouple-pair output of only about twenty microvolts. Thus the change in output would be only about 20/1480 or 1.5 percent of the total output at the peak, and perhaps 0.3 percent at the mean temperature.
Alternatively, commercial circuit elements are available which provide "temperature-compensation" voltage outputs that may be substituted for the output of the reference junction in the reference heat sink. These elements are inexact, demonstrating drift and noise deleterious to extremely accurate measurements, while adding substantially to the cost of the electronics.